Many biological materials show impressive and controllable properties that are determined by their micro and nanostructure. Cellulose fibres extracted from wood and spider silk represent two excellent examples. The main constituent of cellulose fibres is the nano-scale fibril, which has the prospective of being a building block for future high-performance bio-based materials and textiles and/or to provide a template for functional nano-materials. However, processes that enable full utilisation of the potential of the fibrils are yet to be developed. The fibrils in cellulose fibres from wood are organised in a nano-scale lamellar structure having a highly ordered spiralling orientation along the fibre axis. The fibres demonstrate high ultimate strength and stiffness that vary in a wide range depending on the mean fibril orientation. In the tree, the fibril orientation also varies through the thickness of the stem so that the mechanical performance of the tree is maximized.
The fibrils of cellulose fibres can be separated by several methods (1) to produce nano-fibrillated cellulose (NFC) and recently, films and filaments have been manufactured from NFC with promising properties (2, 3, 4). However, the properties obtained are far from the maximum values reported for individual cellulose fibres liberated from wood.
In order to manufacture materials such as fibre-composite materials and textiles there is a need for manufacturing methods for fibres (threads). Specifically, there is a need for environmentally friendly methods using fewer chemicals that can make fibres from bio-based polymers.
Wet spinning of fibres (filaments) is performed in an apparatus where a raw spinning liquid is discharged from a nozzle into a coagulation liquid where a fibre is formed. The raw spinning liquid is commonly prepared by dissolution of constituents into a suitable solvent. These substances typically are non-spherical, which contributes the properties of the final fibres. The spinning liquid is injected through a nozzle, i.e. the spinnaret, into a bath where a coagulation liquid is contained. The injection can be directly into the coagulation liquid by submersion of the nozzle into the coagulation liquid or into the coagulation liquid after passing through a gas such as air.
Typically a drawing roll is immersed at the other end in the spinning bath. The spinning raw liquid discharged from the nozzle (i.e. the spinnaret) is coagulated by the coagulation liquid and thus formed into a coagulated fibre (filament), which can be drawn out of the spinning bath using the drawing roll. The coagulated fibres (coagulated filaments) solidified in the spinning bath are separated from the coagulation liquid, washed, and transferred to subsequent steps such as chemical liquid treatment, drying, and thermal treatment. As an improvement to the process, coagulation liquid has been applied already in the spinnaret, as disclosed in e.g. Patents GB1087212 and U.S. Pat. No. 2,510,135. This process can be performed for single fibres (monofilament) or a multitude of fibres in parallel (multifilament).
In order to meet product requirements regarding fibre mechanical properties such as strength, it has proven to be beneficial to achieve alignment of the constituents within the spun fibre. Furthermore, in order to achieve alignment of the constituents (polymers, fibrils, etc.) there are two main approaches used:    1) The constituents are stretched after coagulating/gelling of the fibre produced has been initiated. This causes the constituents to be elongated and the constituents aligned in the direction of the fibre. There are two routes for this stretching:            a) Mechanically pulling the fibre in its gelled stated; or        b) Using a co-flowing liquid flowing faster than the produced fibre, which pulls the fibre through shear forces excerpted on the developing surface of the fibre;            2) A nozzle designed with a specific geometry, i.e. a nozzle with a contraction, that aligns the constituents in the direction of the fibre produced.
These approaches typically need a sufficiently high viscosity of the spinning liquid to function and the co-flow needs to be significantly faster to achieve stretching. The above approaches (1a, 1b and 2) for achieving orientation of the constituents of the fibre can all be combined. As an example this is disclosed in patent application WO9724476.
In order to achieve a good final product it is preferred to achieve a uniform alignment of the constituents throughout the diameter of the fibres, i.e. the degree of alignment in the core of the fibre should be the same as at the surface of the fibre.
Regarding approach 1a, high shear or strain within the fibre, in the gelled or partly gelled state can cause local weakening of the fibre and thus inferior performance. This effect will be present regardless of type of constituent.
For the case of non-molecular constituents, such as fibrils, shear gradients within the liquid before gelling will also decrease alignment. The effect cannot be avoided and was described by Jeffery (1922) (5). The shear gradient within the liquid will force elongated particles such as fibrils to flip, i.e. rotate, and thus depart from the alignment to the direction of the flow. Furthermore, this effect of fibres forced to depart from the alignment with the flow is significantly enhanced in the presence of a solid surface, which was described by e.g. Holm and Söderberg (2005) (6), which showed that close to a solid surface, elongated particles will tend to orient perpendicular to the flow, i.e. the opposite of the desired alignment along the extent of the fibre.
The detrimental effects of shear gradients on alignment of non-molecular constituents will be present in approach 1b and 2 but not in approach 1a. Molecular substances such as polymers will most likely also be affected by the shear gradient albeit to a lesser degree.
The possibility, although only at higher concentrations, to manufacture fibres from fibrils, e.g. cellulose microfibrils, has been previously exploited and described, e.g. in U.S. Pat. No. 6,248,267, where a process for manufacturing of fibres based on the film-forming ability of the spinning liquid is revealed. This process is based on having a cellulose composition of less than 30 weight percent cellulose or similar matter moulded out from a nozzle into a reaction chamber where it is subjected to a coagulant spray. The preferred embodiments all identify concentrations above 5 weight percent or more. Furthermore, in order to achieve the shear needed in the described reaction chamber the coagulant spray should be co flowing with the spinning liquid with a preferred angle of 20°-80° where 0° corresponds to a coagulant spray in the same direction as the ejection of the spinning liquid from the nozzle.
It should also be noted that all the above mentioned patents and patent applications are characterised by having an axisymmetric design focused on spinning a predominantly cylindrical fibre, and can hence not be extended to form a film.
Specifically the coagulant flows are designed to reach the ejected spinning liquid from all directions, i.e. more or less axisymetrically. Furthermore, as mentioned above these spinning processes all depend on high-viscosity liquids, achieved by having high concentrations of the polymers, fibrils etc., where the surrounding co-flowing liquid has a viscosity at least one order of magnitude lower than the viscosity of the spinning liquid.
As is apparent from the above, there is a need for oriented fibres and films without local weakening or shear gradient. Moreover, there is a need for a method of manufacturing said fibres and films being less dependent on e.g. solvents.